When laying a finishing layer, in particular of bituminous paving material or also of concrete paving material, a relatively small positive setting angle of the screed or smoothing plate is desirable with regard to uniform compaction and good surface planeness. A positive setting angle implies that the front edge of the screed in the working direction, in the region of which the tamper device compacts the paving material, is situated higher above the formation level than the rear edge of the screed in the working direction of laying. The positive setting angle should be maintained as constant as possible, because it influences the paving thickness. The higher the compacting output applied by the tamper device, the higher the local compaction and vice versa. Since operating parameters such as the weight of the screed and the compactability of the layer material are relatively constant, the compacting output is primarily dependent on the stroke and also, though even to a somewhat slighter extent, on the frequency of the tamper device. If the laying rate reduces with the tamper device at a constant stroke and constant frequency then the compacting output increases locally, by means of which, due to the locally increasing compaction, the screed rises, the setting angle reduces undesirably and the paving thickness increases. On the other hand the local compacting output reduces for an increase of the laying rate so that the screed lowers due to the reduced compaction, the setting angle increases undesirably and the paving thickness reduces. Therefore when laying, attempts must be made, largely independently of laying parameters, such as for example the laying rate, to produce a constant compaction so as not to change the setting angle of the screed or as little as possible.
For laying in finishing layer different types of screed are used. A so-called fixed-width screed has a fixed pave width which cannot be changed when laying. The pave width of the fixed-width screed can be increased in steps in that extension parts are fitted to the ends of the fixed-width screed. Each extension part also has a tamper device and a smoothing plate, equipped with an unbalance vibrator if required. Alternatively to the fixed-width screed, an extending screed is used if the pave width has to be varied during laying. In the extending screed extendable and retractable telescopic screed parts are arranged on both ends of a basic screed with a fixed pave width. The basic screed and the telescopic screed parts each have tamper devices. If the maximum pave width of the extending screed is not sufficient, then extension parts, which are each equipped with a tamper device and a smoothing plate can be fitted to the ends of the telescopic screed parts. With all types of screeds the tamper device is operated with the same stroke and the same frequency over the entire pave width.
When laying finishing layers, in particular of bituminous paving material, it is often necessary to lay a finishing layer with a variable paving thickness transverse to the working direction of travel for laying, e.g. when the surface of the finishing layer and/or the formation level has a transverse slope or a straight finish from the shoulder to the center line or a special profile, for example with a concave parabolic profile is to be produced or unevenness in the formation level is to be compensated. Furthermore, the laying rate inevitably varies over the pave width when laying a finishing layer on a traffic roundabout or along a sharp curve. Both effects, i.e. the varying paving thickness or the varying laying rate, lead alternatively or additively for constant stroke and similarly also constant frequency of the tamper device over the pave width to unwanted reactions in the final quality of the finishing layer, such as undesired variations in paving thickness and/or different degrees of compaction over the pave width. These undesired reactions or sacrifices in quality have so far been accepted as unavoidable.
As shown below based on examples, although it is known how to variably control the compaction produced by the tamper device depending on the changing laying parameters, the same control is always applied over the full pave width of the screed, whereby the requirements for a paving thickness varying over the pave width or a laying rate varying over the pave width cannot be taken into account.
With the screed known from DE 4 139 702 C2 the angle between the direction in which the tamper device compacts and, for example, the smoothing plate of the screed is adjustable in order to adapt the compacting output to harder or softer paving material. However, the compacting output is in each case the same over the entire pave width.
With the method known from DE 4 040 029 C1 the frequency of the tamper device of the screed is controlled along a set-point curve according to the actual laying rate in order to maintain the compacting output of the tamper device essentially constant independently of the changes in the laying rate, i.e. to reduce the frequency for a slowing laying rate and to increase it for a quickening laying rate. The stroke of the tamper device remains unchanged over the pave width. Alternatively, with an interruption in laying, the stroke of the tamper device can be changed manually in steps. Since the frequency control of the tamper device however also occurs over the entire pave width in the same manner, the requirements of the paving thickness or the laying rate varying over the pave width are not taken into account.
With the road finisher known from DE 19 836 269 C1 the frequency of the tamper device varies in dependence of changes of the positive setting angle of the screed so that the setting angle proportionally controls the frequency to compensate for negative effects of changes in the laying rate. Here too, the frequency is varied uniformly over the entire pave width.
From the technical information publication “Vögele—Für jede Aufgabe die richtige Einbaubohle” (Vögele—The right screed for each task), published by Josef Vögele AG, Neckarauerstr. 168-228, D-68146 Mannheim, No. 2400/10, printed in February 1997, in particular on pages 4 and 5, tamper devices with hydraulically powered tamper strips are known, whose operating frequency can be varied via the rotational speed of the hydraulic drive and whose stroke can be varied by the manual adjustment of the correspondingly effective eccentricity of an eccentric drive shaft. Furthermore, it is known from this that also the smoothing plate of the screed can be fitted with an unbalanced vibrator, the frequency of which can be varied by control of the rotational speed. Furthermore, the tamper device may produce only pre-compaction of the paving material and, where required, final compaction is carried out by hydraulically powered press strips behind the smoothing plate in the working direction. It is pointed out on page 6 that the stroke of the tamper device determines the maximum possible compression, i.e. the degree of compaction, and the stroke can be adjusted manually in steps for various stroke values, whereby the degree of compaction achieved can also even be increased by increasing the stroke frequency.
With a screed known from US 2002/0141823 A1 the tamper strip, which extends continuously over the pave width, consists of separate tamper strip sections which can be joined together. Each tamper strip section is driven by at least one control cam. The control cams of all tamper strip sections are arranged on a common drive shaft which is supported in the screed and is driven at a required rotational speed. The control cams are phase shifted to one another so that the tamper strip sections pre-compact with a phase offset to one another. The stroke and the stroke frequency are the same for the tamper strip sections so that a uniform pre-compaction output is produced over the working width across the working direction. A remotely actuated variation of the stroke and/or the frequency over the pave width is not elucidated.
With the screed known from U.S. Pat. No. 4,828,428 A two continuous tamper strips are provided at least over the working width of a screed section in front of the smoothing plate of the screed in the working direction, the said tamper strips being actuated by a common drive or separate drives. The stroke and/or the frequency of the tamper strip is the same over the pave width of the screed section. The strokes and the relative timing of the tamper strips can be adjusted. To change the timing either a timing chain is redeployed or exchanged. To change the stroke in steps the relevant eccentric drive components on the drive shaft are exchanged. A variation of the stroke and/or the frequency within the pave width of the screed section and transverse to the working direction is not elucidated.
With the screed known from U.S. Pat. No. 6,019,544 A no tamper devices are provided on the basic screed and the extending screeds, but rather telescopic screed parts are only provided on screed extension parts, which are mounted either in telescopic screed parts or on the outside of the basic screed. These tamper devices with just a tamper strip extending in the outermost marginal region of the finishing layer transversely to the working direction are used for pre-compacting the edges of the finishing layer. Each tamper strip is supported inside in the screed extension part, pivotable about an axis orientated in the working direction, and is actuated on the outer end by a crank mechanism and is pivoted up and down about the pivot axis. The stroke applied to the outer end of the tamper strip can be manually adjusted after removal of the covers and adjustment of a turnbuckle. A paving thickness and/or laying rate varying over the working width is not mentioned.
Finally, it has already been suggested (European Patent Application with the file number 09014516 and prior seniority) that a remote controllable adjustment device is provided for an eccentric stroke drive of a tamper strip of the tamper device on a screed, with which the stroke can change in dependence of the changing laying parameters even in laying operation over the entire pave width.